But teaching unions ASCL and NASUWT have defended Ms Davidson’s tenure and former government adviser Prof David Egan has slammed “infantile analyses” of previous regimes.

Prof Reynolds, who works at the University of Southampton but lives in Rhondda Cynon Taf, believes Wales should have learned from other countries in the wake of devolution.

“Across the world, countries did the demand-side changes with parents and publishing data for them,” he said.

“They did the supply-side by national programmes aimed at helping teachers to get better. We did neither. We did curriculum change in Wales but only some of it rooted properly.

“History will record Ms Davidson was a great curriculum innovator. It will also record that she gave us a decade when the things that implement curriculum – teachers and teaching – were forgotten.”

Ms Davidson is credited with introducing two of Wales’ sector-leading policy initiatives in the form of the Foundation Phase and Welsh Baccalaureate. Education experts have high hopes for both but their relative infancy means meaningful results will take time to filter through.

Dr Philip Dixon, director of teaching union ATL Cymru, said the “national humiliation” of Wales’ worsening position in Pisa’s world survey was proof that problems are deep-rooted.

And he believes the worrying state of basic skill levels portrayed last month by education watchdog Estyn was not formed overnight.

Responding to Ms Davidson’s wide-ranging interview with the Western Mail, Dr Dixon said: “The leaking roofs, faulty windows and Victorian loos of many Welsh schools are a lasting monument to her seven-year reign as education minister.

“While New Labour were spending millions on the English school system, transforming buildings in the process, the Welsh education system was being starved of cash. The target of making every school fit for purpose by 2010 was quickly and quietly dropped when it became apparent that it would be missed by a mile.”

In 10 years, Welsh teenagers have gone from achieving an above-average percentage of five A* to C grades at GCSE to a percentage significantly below average. Wales attracts fewer top grades at A-level and international comparators rank the nation’s secondary school children at the bottom of the UK.

Dr Dixon said a “toxic legacy” of underfunding in Wales – with the gap in pupil spend rising by £400 in the seven years from 2000 – was having an effect.

He said: “Money might not be the only problem, but it would be remarkable if a funding gap of this magnitude has had no effect on the gap in literacy levels, GCSE and A-level attainment and the dreaded Pisa results. It’s not the vision but the delivery that has been problematic, the ‘how’ should have received as much attention as the ‘what’ – and it didn’t. The hard facts and the disturbing data cannot be dismissed if we are to have a balanced picture.”

Despite government ministers having only limited control over local authority expenditure, questions remain as to why sliding delegation levels were not tackled sooner.

Under Education Minister Leighton Andrews, the Welsh Local Government Association has made a commitment to increase council money passed directly to schools by 10%.

Speaking to the Western Mail, Ms Davidson last month said the level to which different elements are funded often depends on local circumstances, which can in turn lead to variation.

Prof Egan believes that devolved Wales has been “extremely fortunate in the quality and vision of its Education Ministers” and we should be confident that their “building blocks” will deliver results.

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